1. Field of the Invention
The present invention relates generally to pulse modulators, and more specifically, to a consecutive edge modulator having centered pulses and noise shaping.
2. Background of the Invention
Consecutive edge modulators (CEMs) are desirable in applications such as digital-to-analog (D/A) converters and power output circuits due to the effectively doubled control update rate per pulse. Since the pulse output is controlled with respect to both the leading edge and the trailing edge, the control function is more responsive, leading to a lower operating frequency for a given required response.
In particular, in applications requiring a highly stable and accurate output, the CEM output circuit is driven by a noise-shaping modulator that pushes the conversion “noise” due to the discrete nature of the CEMs transfer function to the high end of the frequency spectrum, where the output filters can effectively remove the noise. In such applications, the nature of the alternating control function of the CEM requires inversion of the CEM control input signal provided by the output of the noise shaping modulator. However, the alternating control function also causes the noise-shaping correction to break down, as the shaping correction causes large changes in edge position between adjacent pulses.
U.S. Pat. No. 5,708,433 describes a CEM methodology that applies a non-linear correction function of alternating polarity to the noise-shaping modulator and also mentions another prior technique of applying separate noise shaping to the positive and negative edge controls (interleaved noise-shaping). The technique disclosed in U.S. Pat. No. 5,708,433 represents an improvement over interleaved noise-shaping in that the higher effective oversampling rate of the CEM is also advantageously applied to the noise shaping. The above-mentioned technique observes the output history of the CEM pulse modulator, applies a non-linear correction with alternating polarity and effectively introduces the correction to the input by combining the corrections with the input signal in the noise-shaping modulator.
However, the above-mentioned technique also introduces an artifact: the response to a DC level at the noise-shaper input has a component at the quantizer output that causes the rising edge and falling edge portion of the CEM output pulses to differ in expected value. The difference between the pulse portions skews the available dynamic range, thus shortening it. Further, positive feedback causes the edge positions to “creep” forward in time. The above-described CEM also of necessity includes delays in the forward signal path due to the pulse history required to provide the look-ahead operation.
Therefore, it would be desirable to provide a CEM method and apparatus that includes noise-shaping applied at the full output edge rate with substantially centered pulses and no movement of the pulses in time. It would further be desirable to provide such a CEM apparatus that does not require storage of a history of the pulse output and does not have substantial delay in the forward signal path.